Have you ever had an experience like this? When one of your dogs dies, you feel sad, but the other dogs also have changes. They are no longer lively and will even look at the place where the passing dog used to be, or at the pictures you have put up, or at the pet urn. They feel sad just like people do, and in their own way they are remembering their best friend.
Some dogs may lose their appetite. If you notice that they suddenly lose their appetite, and don't wag their tail as much as they used to when see the food, you don't have to worry too much, this may be one of the signs and symptoms that they are feeling sad and grieving over the loss of another companion.
Some may sleep more than usual. They want to sleep all day and do nothing, sometimes they will gaze at their pet urn all day. You may also observe that their behaviour can become aggressive. They don't want to explain the pain they feel after losing a sibling they went with. You may hear them making calls or vocalisations for their deceased dog. This may indicate that they miss their pet very much.
They may also feel frustrated when they can no longer hear another dog. They don't want to be disturbed and can sometimes be irritated. Their reactions to the loss depend on how close they were to the other dog who died.
Your emotions will also affect how they feel as they spend time together day after day. With this their departure, your sadness and loneliness will spill over. The longer you feel sad, the longer they feel your pain, and it continues on a daily basis.
As their parent, you certainly wouldn't want them to dwell on their grief all the time, even if you know it's a very sad fact indeed. Perhaps a pet urn would be another kind of companionship, an alternative, a presence to make things easier for each other.